Solubility Test of Bitumen: Procedure and 99% Limit

The solubility test of bitumen is a purity check. Real bitumen dissolves almost completely in certain solvents. Dirt, stone dust, carbon and many contaminants do not. Dissolve a weighed sample, catch whatever is left on a filter, weigh it, and you know what share of the material is actual binder. It is one of the quiet acceptance tests on most paving bitumen specifications, and it is simple enough to explain in a few steps.

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Quick facts

Standards: ASTM D2042 (trichloroethylene or toluene), AASHTO T 44, IS 1216 in India.
Sample: about 2 g, dissolved in 100 mL of solvent.
Filter: a glass-fiber pad in a Gooch crucible.
Typical limit: at least 99.0% soluble for paving grades in IS 73.
Applies to: binders with little or no mineral matter. Filled products need other methods.

Why purity matters

Only the soluble fraction coats and binds aggregate. Insoluble matter just takes up weight that the buyer paid for as binder. Worse, it can be a sign that the product was overheated in a tank and partly coked, or that it picked up fines in a dirty tanker. The test does not say what the impurity is, only how much there is, but a low result is a clear signal that something is off.

Equipment

A 125 mL Erlenmeyer flask.
A Gooch crucible with a glass-fiber filter pad, plus a filter flask and tube for vacuum filtering.
An oven that holds 110 ± 5 degrees C and a desiccator for cooling.
An analytical balance.
The solvent, handled in a fume hood with gloves and eye protection.

Step-by-step procedure

Prepare the crucible. Seat the pad, wash it with solvent, dry it in the oven, cool it in a desiccator and weigh it (A).
Weigh the sample. Put about 2 g of bitumen into the tared flask and record the exact mass (B).
Dissolve. Add 100 mL of solvent a little at a time while swirling until no lumps of undissolved sample remain. Cover the flask and let it stand for at least 15 minutes.
Filter. Pour the solution through the crucible with light suction, then rinse the flask with small amounts of clean solvent until everything is on the pad and the drip is colorless.
Dry and weigh. Let the solvent evaporate, dry the crucible in the oven, cool it and weigh it (C). Repeat drying until the mass stops changing.

Calculating the result

Percent insoluble = [(C − A) ÷ B] × 100
Percent soluble = 100 − percent insoluble

Example with made-up numbers: a 2.000 g sample leaves 0.012 g on the pad. Insoluble = 0.012 ÷ 2.000 × 100 = 0.6%. Soluble = 99.4%, which clears a 99.0% minimum.

Typical limits

IS 73 (India): 99.0% minimum for VG-10 through VG-40.
AASHTO M 226 (older US AC grades): 99.0% minimum.
Natural asphalts: products like lake asphalt contain mineral matter by nature and are specified under their own rules rather than the straight-run paving limits.

Trichloroethylene, toluene and safety

Trichloroethylene was the traditional solvent because it dissolves bitumen quickly and does not burn easily. It is also toxic, so the standard calls for good ventilation and careful handling, and the current ASTM edition allows toluene as an alternative. Results from different solvents should be compared with care, so the solvent used belongs on the test report.

Solubility vs ignition oven

Do not confuse this test with the binder content tests run on finished mix. The ignition oven burns binder off a mix sample to find how much was there. The solubility test is run on the binder alone, before it goes into the mix, to check that the binder itself is clean. It sits alongside the flash point, viscosity and ductility checks on a typical certificate.

Questions people ask

What does the solubility test of bitumen show?

It shows how much of a sample is actual bitumen and how much is something else, such as mineral matter, carbon or other contamination. The soluble part is the binding material; the insoluble residue adds nothing to how the binder glues aggregate together.

What solvent is used?

Trichloroethylene has long been the standard solvent in ASTM D2042 and IS 1216. The current ASTM D2042 edition also allows toluene. Trichloroethylene is toxic, so the work is done in a fume hood with good ventilation.

What is the minimum solubility for paving bitumen?

IS 73 requires at least 99.0% solubility in trichloroethylene for VG-10, VG-20, VG-30 and VG-40. The older US viscosity-graded asphalt cement specification, AASHTO M 226, also set 99.0% as the minimum.

How is the result calculated?

Percent insoluble = (mass of residue on the filter / mass of sample) x 100, and percent soluble = 100 minus that. The residue mass is the crucible, pad and residue after drying minus the crucible and pad weighed before the test.

Why would a binder fail solubility?

Common reasons are mineral fines picked up in storage or transport, coked material from overheating, or a product that naturally contains mineral matter, such as some natural lake asphalts. A failing result means the delivered material is not the clean binder that was specified.

How big is the sample?

About 2 g of bitumen is dissolved in 100 mL of solvent in a 125 mL flask, then filtered through a glass-fiber pad in a Gooch crucible.

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